"This release incorporates major changes to the core of the Xfce desktop environment and hopefully succeeds in fulfilling a number of long time requests. Among the most notable updates is that we have ported the entire Xfce core (Thunar, xfdesktop and thunar-volman in particular) from ThunarVFS to GIO, bringing remote filesystems to the Xfce desktop. The panel has been rewritten from scratch and provides better launcher management and improved multi-head support. The list of new panel features is too long to mention in its entirety here. Thanks to the new menu library garcon (formerly known as libxfce4menu, but rewritten once again) we now support menu editing via a third-party menu editor such as Alacarte (we do not ship our own yet). Our core libraries have been streamlined a bit, a good examplle being the newly introduced libxfce4ui library which is meant to replace libxfcegui4."

1. I mention Unity because we're talking about something that will be shipped as default with Ubuntu, which is the main and most used Linux distribution. Ubuntu is so popular that the complexity of just having it as default is enough to cause changes in the entire desktop ecosystem.

No not really. There are lot more fedora, archlinux and opensuse users.
Also for Ubuntu to actually affect upstream code or default configuration, it would mean they actually have to contribute to upstream which ubuntu has always been against. Ubuntu's policy is "upstream works for us".
Ubuntu can stop existing this second and nothing will change.
Redhat and opensuse can stop existing and no more gcc/glibc/gnome/openoffice/wayland/xorg...